U.S. Pat. No. 3,855,638 to Pilliar discloses a surgical prosthetic device that consists of a solid metallic material substrate with a porous coating over at least a part of its surface. The coating has a thickness between one hundred to one thousand microns. The coating is formed from metallic powder sized between -100 to +325 mesh. The patent discloses production of the coating by using a slurry of metallic powder suspended in an aqueous solution with organic binders. The particle size of the metallic powder and conditions of formation of the porous coating are controlled to provide the desired interstitial pore size, porosity, strength and depth of coating. Both the substrate and powder are sintered to achieve metallurgical bonding between engaged metal particles and between the metal particles and the substrate. The disclosure states it to be essential that the porosity of the surface coating not exceed about 40% and be at least about 10%. It states that a porosities above 40% the overall mechanical strength falls below the required level.
U.S. Pat. No. 3,852,045 to Wheeler, Sump and Karagianes, discloses a porous metallic material with interconnected voids, again directed to tissue ingrowth purposes. Voids or pores are produced in the metallic material by use of expendable void formers. The composite material is treated by high energy rate forming pressures to densify its structure prior to removal of the expendable void former. Substantial thicknesses of the coating on substrate metallic elements are disclosed.
While the products resulting from the systems taught in U.S. Pat. No. 3,852,045 have performed satisfactorily, the practical application of the system is severely limited by both the expense and availability of equipment for the required high energy rate forming steps. Furthermore, such steps are of questionable value when attempting to produce a relatively thin porous coating on implant elements, since the high pressures to which the elements would be subjected might result in structural damage to them.
According to the present invention, relatively thin porous metallic coatings are produced about selected surface area configurations on a prosthesis by forming the coating about the surface, using a blended mixture of primary and secondary particles. The primary particles are either made from a material identical to or metallurgically compatible with the metallic surface being coated. The secondary particles are made of an expendable material. Both are heated and pressed in place about the surface to effect metallurgical bonding between engaged primary particles as well as between the surface area and the primary particles in contact with it. The expendable material is subsequently removed to achieve controlled porosity throughout the coating.